The efficiency of an End Fed Half Wave (EFHW) antenna transformer can vary depending on several factors, including the design of the transformer, the quality of components used, and the surrounding environment. However, with a well-designed and properly installed transformer, the efficiency can be quite high.
The primary purpose of an EFHW antenna transformer is to match the impedance of the antenna to the impedance of the transmission line or the radio transmitter/receiver. This impedance matching helps in maximizing power transfer and minimizing signal reflections, which can affect efficiency.
Efficiency is typically measured as the ratio of the power radiated by the antenna to the power delivered to the antenna. Ideally, you would want the power radiated to be as close as possible to the power delivered. In practical scenarios, losses occur due to various factors such as resistive losses in the transformer, radiation losses, and mismatch losses.
To improve the efficiency of an EFHW antenna transformer, here are a few considerations:
1. Transformer design: A well-designed transformer with high-quality components and low-loss materials can help minimize losses.
2. Impedance matching: Proper impedance matching between the antenna and the transmission line helps to minimize mismatch losses. This can be achieved through careful design and tuning of the transformer.
3. Installation and environment: The physical installation and the surrounding environment can have an impact on the efficiency. Avoiding nearby obstructions, minimizing interference sources, and proper placement can help optimize performance.
4. Loss reduction techniques: Using techniques such as using low-loss transmission lines, reducing resistive losses in the transformer, and minimizing other sources of losses can improve overall efficiency.
It's important to note that the efficiency of an EFHW antenna transformer can never be 100% due to inherent losses. However, with careful design, installation, and tuning, it is possible to achieve high efficiencies, often in the range of 80-90% or even higher. The graph below is a measurement of efficiency from our 2xFT240-43 models. There is some loss in the test rig, however its relatively insignificant.
Efficiency is measured using two transformers wired together back-to-back. They have to be of the same toroid type, mix and winding configuration. Then we connect a calibrated NanoVNA to each transformer and measure the return loss in DB through both transformers. This gives a DB negative gain value with is divided by 2 to give a single transformer loss figure. This is then converted into a percentage of efficiency.
A point to note is any losses in the transformer is converted into heat. This is why when you need a high power broad band End Fed Half Wave transformer you should be looking for a larger more efficient design.
Lower power sub 100w SSB or 20w Data transformers do not need very large cores. QRP transformers can be very small. This is why we produce a range of EFHW transformers and antennas.
Update.
Since publishing this blog we have tested several of the popular cores for End Fed Half Wave transformers.
These are our efficiency test to date.
For Fair-Rite FT240-52 cores as an auto transformer we measured upto 91% efficiency on the 40m band. Also, this stack is quite efficient on the 10m band at 82% or .88db loss.
For the Fair-Rite 2643251002 core we measure efficiency up to 90% on 80m band and 86% at 10m band making this an excellent all round core for 300w SSB and 100w data/cw.
Fair-rite 2643101902 core measured at 93% at 80m and 90% 10m band, this is an excellent core for 100w SSB and 35w data uses.
And the Fair-Rite 2643625002 core, a QRP core with 82% efficiency from 80m to 10m.
I hope this information has proved useful.